A glow plug used for assisting the starting of a diesel engine includes a heater that generates heat by conduction and that is retained on the front end side of a main metal shell. The glow plug is used with the front end portion of the heater jutting into the engine. The main metal shell has a shaft hole into which a metal conductive member (such as a bar-like center wire) having electrical continuity with a heater terminal and extending toward the rear end side in the axial direction is inserted. The rear end portion of the center wire protrudes from the rear end surface of the main metal shell. Further, the rear end portion of the center wire is fixedly enclosed by a metal terminal member by crimping or threaded engagement for connection with an external device, thus establishing electrical continuity between the center wire and the terminal member.
In the glow plug of such a configuration, the gap between the rear end portion of the main metal shell and the center wire is sealed with an O-ring in the shaft hole so as to ensure airtightness between the inside of the glow plug (main metal shell) and the outside on the rear end side. Further, an insulating member for ensuring insulation between the main metal shell and the center wire is disposed on the rear end side of the O-ring between the rear end portion of the main metal shell and the center wire (e.g., Patent Document 1).